This invention relates broadly to the art of continuous casting, and more particularly to a device which combines electromagnetic continuous casting and mold-type continuous casting.
The following discussion is especially directed to aluminum metals, as this invention is of particular advantage when used therewith. In this application, the term "aluminum metals" is intended to mean aluminum metal and alloys containing at least 50% aluminum.
The method now in greatest use in continuous casting of aluminum ingots is the direct chill or "DC" casting method disclosed by Ennor in U.S. Pat. No. 2,301,027 issued Nov. 3, 1942. This method employs a mold in the form of an open-ended sleeve or shell having a straight bore therethrough, with a surrounding sprayer which directs cooling fluid, such as a water spray, against the mold and also directly against the embryo ingot as it is withdrawn from the mold. The operation of the DC process forms a solidified surface around the ingot while it is within the mold, and then exposes that surface to the cooling water spray beneath the mold. The mold is relatively long so that a head, typically 2 to 4 inches, can be built up in the mold above the freeze zone. The mold, operated with a constant head, provides lateral support for the molten metal.
More recently, work has been done with electromagnetic casting systems, one of which is described in U.S. Pat. No. 3,605,865 to Getselev. This system has not yet been sufficiently developed to have extensive commercial use. Basically, in an electromagnetic casting system, molten metal is first introduced at a controlled rate onto a bottom block, or pan, located within a loop-shaped inductor to form an embryo ingot. The bottom block is lowered at a controlled rate with metal flow being controlled in accordance with this rate to form an ingot. The molten metal is confined laterally by electromagnetic forces generated by an alternating current in the inductor. The molten metal is thus formed into a shape similar to but smaller than the inductor. The emerging ingot is solidified in this shape by the application of a coolant such as water.
In most electromagnetic casting systems, there is an electromagnetic shield, or screen, located inside the inductor and arranged coaxially therewith. The shield is normally made of a non-magnetic, electrically conductive metal having a relatively high resistance such as stainless steel. The shield serves to attenuate the magnetic field of the inductor upwardly thereby lessening the electromagnetic forces restraining the ingot at the top as opposed to those at the lower edge of the shield. The advantages of such a shield are fully described in U.S. Pat. No. 3,605,865 to Getselev and the information in that patent is incorporated by reference herein. In this respect, the shield only attenuates the electromagnetic field, it does not eliminate it and Getselev points out in column 2, lines 31-42 in U.S. Pat. No. 3,605,865 that at frequencies of 1,000 to 2,000 Hz. the shield is made of a non-magnetic steel (such as stainless steel) having a high specific resistance, while at a frequency of 50 to 500 cycles per second, the shield can be made of aluminum or copper. The reason for these limitations is that above 500 cycles per second, aluminum and copper shields, because they are such good conductors, substantially block out the electromagnetic field within them so that the electromagnetic field within the shields would provide no restraining force for the molten metal.
Summarizing electromagnetic continuous casting systems, they are similar to the normal DC casting systems with the exception that they replace an aluminum mold of the DC system with an inductor and shield for creating an upwardly attenuated electromagnetic field to support the column of molten metal.
Still a third type of prior-art continuous casting that is pertinent to this invention is "low-head" continuous casting. Low-head continuous casting has been accomplished in the past, however, it is not widely used commercially because it is too difficult to start and control.
Low-head casting is basically performed without the aid of a mold or an electromagnetic inductor to laterally restrain the molten metal; thus, the name, low-head casting. The concept involves balancing heat transfer with molten metal feed such that as surface tension of the molten-metal's meniscus forms an upper edge of an ingot the molten metal is frozen. In other words, the freeze zone of the ingot is very close to the bottom of the miniscus of the molten metal so that it is not necessary to provide an external force, such as a mold or an electromagnetic field, to hold the molten metal in position before it is frozen. Such molding requires a very critical balancing of heat transfer, lowering rate and molten-metal feed. A possible undesirable result of an improper balance is a metal spill, or explosion, which can be dangerous to operators. It is thought that this is a main reason that low-head continuous casting has not been commercially successful in the past. Thus, it is an object of this invention to provide a system for low-head casting which can be adequately started and controlled.
It is yet another object of this invention to provide a continuous casting system and method which produces relatively high-quality ingots, especially with respect to surface quality with high solidification rate of surface areas.